1. Field of the Invention
The present invention relates to a projection optical system, and more specifically to an image projection apparatus having, for example, a digital micromirror device or a LCD (liquid crystal display), as a display device and to a projection optical system that is mounted in the image projection apparatus and that perform enlargement projection of an image formed on the surface of the display device onto the screen surface.
2. Description of Related Arts
As an image display for use in digital satellite broadcasts at home and a home theater, low-cost and high-performance image projection apparatuses (that is, projectors) having a digital micromirror device or an LCD as a display device have been becoming increasingly widespread in recent years. Thus, there has been a great demand for a slim rear projector that does not occupy too much space even in a small room, which requires a wider-angle projection optical system. Moreover, the volume of information displayed has been considerably larger than before, and the display device has been provided with an increasingly higher resolution than before accordingly, thus leading to a demand for smooth, clear, and high-quality images.
To achieve a low-cost rear projector with a high resolution, so-called pixel shift has been conventionally practiced by which an image projected on the screen surface is shifted periodically by a small degree (for example, one half of a pixel) so as to increase the apparent number of pixels. The practice of pixel shift causes pixels to be superimposed on one another due to after-image, which makes the contour of the pixels projected on the screen surface less outstanding, thereby permitting display of a smooth image. Moreover, due to its ability to increase the volume of information displayed on the screen surface without changing the number of pixels of the display device, the pixel shift is very effective in achieving a higher-grade image through an improvement in the resolution. As a projector employing this pixel shift, Patent publications 1 and 3 propose those which achieve pixel shift by way of slightly decentering a mirror while Patent publication 2 proposes one which achieves pixel shift by way of slightly decentering a flat glass.                Patent publication 1: JP 4-319937 A        Patent publication 2: U.S. Pat. No. 5,237,399        Patent publication 3: JP 7-49477 A        
With the projector constructions proposed in Patent publications 1 to 3, image projection is performed with an optical power possessed by a refractive optical system. The refractive optical system typically has lateral chromatic aberration which deteriorates the sharpness level (resolution feeling) of an image. In pixel shift, when the projection optical system has a predetermined value of lateral chromatic aberration or more, for example, a white line, if displayed, is recognized as a double line. This will be described in detail.
A normal display device surface has matrix structure with a square array of square pixels whose sides are parallel to the vertical and horizontal sides of the image. For pixel shift, “oblique pixel arrangement” is more suitable. In the oblique pixel arrangement, as shown in FIG. 8A, a layout is provided such that the sides of square pixel tilts obliquely by 45 degrees with respect to the vertical and horizontal sides of the image. The length of a diagonal line of each pixel corresponds to a pixel pitch p of horizontal pixel arrays (A1, A2, A3, . . . ; B1, B2, B3, . . . ; C1, C2, C3, . . . ; D1, D2, D3, . . . ). Therefore, when pixel shift (d: amount of pixel shift) is performed vertically upward by a half pitch (=p/2) with respect to the horizontal pixel arrays, the pixel arrays before the shift (A1, A2, A3, . . . ; B1, B2, B3, . . . ; C1, C2, C3, . . . ) shown in FIG. 8A and the pixel array after the shift (B1′, B2′, B3′, . . . ; C1′, C2′, C3′, . . . ; D1′, D2′, D3′, . . . ) shown in FIG. 8B are alternately located on the same straight line as shown in FIG. 8C.
Performing pixel shift by linking to switching of image display so as to display a former frame with the pixel arrays before the shift (A1, A2, A3, . . . ; B1, B2, B3, . . . ; C1, C2, C3, . . . ; D1, D2, D3, . . . ) and display a latter frame with the pixel arrays after the shift (A1′, A2′, A3′, . . . ; B1′, B2′, B3′, . . . ; C1′, C2′, C3′, . . . ; D1′, D2′, D3′, . . . ) results in doubling the amount of information due to visual superimposing of the pixels, thereby providing a high definition projected image. For example, with the pixel array C1, C2, C3, . . . of the former frame and the pixel array D1′, D2′, D3′, . . . of the latter frame, one line of image composed of a pixel array of C1, D1′, C2, D2′, C3, D3′, . . . can be displayed.
Even when pixel shift is performed with the oblique pixel arrangement as described above (FIGS. 8A to 8C), if the projection optical system has a refractive surface, lateral chromatic aberration occurs on the screen surface Si. FIG. 9A shows one example of display of colored light of G and (R+B) before and after the shift. In FIG. 9A, a vertical line portion is displayed in G while a horizontal line portion is displayed in R and B. The intensity distribution of G and (R+B) as shown in FIG. 9B is obtained, shift of which corresponds to lateral chromatic aberration (component in the direction of pixel shift). The intensity distribution is mountain-shaped due to the oblique pixel arrangement; as a result, a deep valley is formed between the peak of G and the peak of R and B, thus resulting in a double-line-like appearance of the projected image, which leads to deterioration in image quality (resolution feeling in particular). More favorable imaging performance (coma, spherical aberration, astigmatism and the like), other than lateral chromatic aberration, which the projection optical system possesses results in more clear display of the double line, as shown in FIG. 9C. In addition, the same problem of a double line also appears in the case of pixel arrangement with square arrays having a small aperture ratio. Even when a double line is not formed, lateral chromatic aberration damages the resolution feeling of an image, thus reducing the effect of pixel shift.